During the p-n junction formation, when an electron diffuses from n → p, it leaves behind an:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → ionised donor on n-side
When a p-n junction forms, electrons from the n-side diffuse to the p-side, where their concentration is low. Each free electron on the n-side came from a donor atom (for example a pentavalent impurity in silicon).
When the electron leaves, its donor atom stays behind as a fixed positive ion, an ionised donor, on the n-side. In the same way, a hole diffusing from p to n leaves an ionised acceptor (negative) on the p-side. These fixed ions form the depletion region.
Option (1) ionised acceptor on n-side — Incorrect. Acceptors are on the p-side; they are left ionised by the holes, not by the electrons.
Option (2) ionised donor on n-side — Correct. The electron leaves its donor atom as a positive ion on the n-side.
Option (3) ionised donor on p-side — Incorrect. Donor atoms are on the n-side, not on the p-side.
Option (4) ionised donor on n-side and p-side both — Incorrect. The p-side has acceptors, not donors.
NCERT (2026-27):
p-n Junction Formation – “… When an electron diffuses from n → p, it leaves behind an ionised donor on n-side. This ionised donor (positive charge) is immobile as it is bonded to the surrounding atoms.”